An in vitro slice preparation was utilized to study quantitatively the action of ethanol on synaptic excitation in the hippocampus from guinea pig brain. Ethanol, 60 to 300 mM, reversibly reduced synaptic activation. The dose-response curve for the reduction of the field potential of antidromically activated granule cells was greater by a factor of 2. The results indicate that the hippocampal slice should prove a useful model for investigating the mechanisms of ethanol's effects on synaptic transmission in brain.